The present invention relates to an outer shroud assembly for use in a gas turbine engine. More particularly, the present invention relates to a ceramic shroud assembly including a metal clamp ring shrink fitted around a ceramic shroud ring, where the metal clamp ring is configured to attach to a turbine engine casing.
As gas turbine engine operating temperatures have been elevated in order to increase engine efficiency, many metal alloy (“metal”) gas turbine engine components, such as a shroud or rotor blade, have been targeted to be replaced by ceramic equivalents. Ceramic materials are able to withstand higher operating temperatures and require less cooling than metals. Ceramic components are also generally less sensitive to thermal expansion than metal components because ceramic materials generally exhibit a lower coefficient of thermal expansion (CTE) than a metal.
In one type of gas turbine engine, a static shroud ring is disposed radially outwardly from a turbine rotor, which includes a plurality of blades radially extending from a disc. The shroud ring at least partially defines a flow path for combustion gases as the gases pass from a combustor through turbine stages. There is typically a gap between the shroud ring and rotor blade tips in order to accommodate thermal expansion of both components during operation of the engine. The gap decreases during engine operation as the rotor blades thermally expand in a radial direction in reaction to high operating temperatures. It has been found that ceramic rotor blade tips experience a reduced radial displacement as compared to metal rotor blades because ceramic materials posses a lower CTE than metals. As a result, in a gas turbine engine incorporating ceramic rotor blades, there is a relatively large gap (or clearance) between the blade tips and the shroud ring. It is generally desirable to minimize the gap between a blade tip and shroud ring in order to minimize the percentage of hot combustion gases that leak through the tip region of the blade. The leakage reduces the amount of energy that is transferred from the gas flow to the turbine blades, which penalizes engine performance.
In order to minimize losses induced by relatively large clearances between rotor blade tips and static shroud rings, some gas turbine engines are able to reduce the clearance by utilizing a ceramic shroud ring rather than a metal shroud ring. A ceramic shroud ring undergoes less thermal distortion during engine operation than many metal shroud rings due to the higher stiffness, lower CTE, and higher thermal conductivity of ceramic materials as compared to metals. Furthermore, a ceramic shroud requires less cooling than a metal shroud because ceramic material is capable of withstanding higher operating temperatures.
In contrast to many metal shroud rings, it is difficult to attach a ceramic shroud ring to a metal gas turbine engine casing because the ceramic material exhibits a low ductility and a lower CTE than the metal casing. In general, stresses may generate at an interface between a ceramic component and a metal component because the ceramic and metal components react differently to the same temperature.